This invention relates to a process for producing polycarbonate oligomers.
As is well known, the production of polycarbonate usually comprises preparing a polycarbonate oligomer of a relatively low molecular weight as an intermediate, and polymerizing the oligomer to a polycarbonate of a high molecular weight. In the conventional production of polycarbonate oligomers, either a tubular reactor or a tank-type reactor is used.
For example, Japanese Patent Publication No. 4352/66 discloses a process for continuously producing polycarbonate oligomers using a packed column-type tubular reactor. This process has the advantage that the reaction time can be shortened as compared with a process using a tank-type reactor. However, the process of the Japanese patent publication cannot easily remove the heat of reaction, and also has the defect that because the flow of the reaction mixture does not satisfactorily reach a steady state, the reproducibility of the reaction is poor, and consequently, the quality of the polymer obtained in the subsequently polymerization satage is not constant.
Japanese Laid-Open Patent Publication No. 14297/72 also discloses a process for preparing polycarbonate ogligomers using a packed column-type tubular reactor, in which phosgene is used together with an inert gas in order to remove the heat of reaction easily. This process, however, has the defect of requiring a gas-liquid separator, and is still not free from the defect that the quality of the final polymer is not constant because of the poor reproducibility of the reaction.
Japanese Patent Publication No. 21460/71 discloses a process for producing polycarbonate oligomers using a tubular reactor having an orifice plate inserted in it. According to this process, the length-to-diameter ratio of the reaction tube must be considerably high. Moreover, since only the phosgenation reaction alone tends to proceed, the resulting oligomer has a low molecular weight and contains a large amount of the terminal --OCOCl group. Thus, the decomposition of the --OCOCl group is liable to occur in a reaction of forming a polymer by polymerizing the oligomer.
Japanese Laid-Open Patent Publication No. 116195/74 discloses a process for producing polycarbonates, which comprises separately introducing gaseous phosgene at a high speed and an alkaline aqueous solution of a dihydroxy compound at a lower speed in the absence of an organic solvent into a tubular reactor equipped with a bi-component nozzle to form a polycarbonate oligomer in it, associating the resulting aqueous reaction mixture with an organic solvent downstream of the tubular reactor, introducing the associated stream into a second tubular reactor, and polymerizing the oligomer in it. However, the removal of the heat of reaction by merely jetting out the gaseous phosgene to cause the flow of liquid is insufficient and it is also difficult to inhibit side-reactions.
The processes described above which rely on the use of tubular reactors have the defect that the heat of reaction is difficult to remove, the length-to-diameter ratio of the reaction tube must be made high, and many reactors must be installed for mass production. Processes which involve the use of tank-type reactors in order to remedy the defects associated with the tubular reactors are known. However, these processes have the defect of reduced rates of reaction. Presumably, the reduced rate of reaction is ascribable to the fact that when a tubular reactor is used, the reaction between a dihydroxy compound and phosgene takes place vigorously at the inlet of the reactor, whereas in the case of a tank-type reactor, the reaction proceeds slowly in the entire inside of the reactor. Furthermore, since the reaction is carried out slowly in the tank-type reactor, the removal of the heat of reaction is relatively easy, but on the other hand, more decomposition reactions occur, and the yield of the final product decreases.
In an attempt to prevent such a decrease in the rate of reaction in a tank-type reactor, British Patent 1,118,146 discloses a process in which the reaction is carried out in the emulsified state. However, this reaction has the defect that the decomposition of phosgene becomes rapid, and the yield of the product decreases.
It is an object of this invention to provide a novel process for producing polycarbonate oligomers, which is free from the aforementioned defects of the conventional processes for producing polycarbonate oligomers using a tubular or tank-type reactor.